Depends on what you're talking about. If you start a tweet with @username, only that person you mention will see it. Now, if you place @username anywhere else in the tweet, everyone who follows you will see that tweet in their timeline.
Any tweets you write (which don't begin with an @username) will appear on your timeline for all of your followers to see. Anyone who also visits your profile (providing you don't have protected tweets) will also see your tweet.
the creator of robloxes username is builderman56
None of them are technically the coolest because everyone one has favourites. Mine is personally Dr. Spencer Reid. (as you can see in my username)
Everyone already can see it my username on the game is leopard girl please add me as your sponsor to receive a special prize or you can message me if you need any help!
Whomever has your username and password can see your email, unless you have your profile viewable to everyone or a select group of people. It's illegal for someone to hack your page.
delete your account if you can then make a new one
Not EVERYONE hates the Jonas Brothers, but I do.(Have you seen my username?) (no we havent seen your user name) DUUUH!
Yes you can change your username by going to your settings and changing the username displayed in the username box. See the related link for an article on this in the Twitter Help Center.
Your username is displayed in the top-left corner - directly under the word 'Answer'
if your username is searched enough it will show up
You'll see your mentions if you click on your username or you could click on your @username and you'll see people who sent you messages